This invention pertains to a process of making alkylidene bis(dibromophenols) also known as tetrabromobisphenols, and more particularly to a process to produce these compounds with higher purity while simultaneously reducing the formation of alkyl bromide by-products.
Tetrabromobisphenols are readily available and widely used compounds, typically prepared by brominating bisphenols in methanol or ethanol. During preparation, the prior art processes routinely brominate the solvent as well as the biphenol. The alkyl bromides formed in the side reaction, for example methyl bromide, are separated from the tetrabromobisphenol and are presently sold. However, due to regulation by the United States government and possibly others, alkyl bromides will soon be the subject of increased environmental control and may altogether be removed from industrial production. As a result, many of the current methods to produce tetrabromobisphenols will not be commercially viable unless the alkyl bromide by-products are converted to other salable products. But given the relatively large volume of tetrabromobisphenols produced worldwide, it is unlikely that this will be an economical alternative. Therefore, another process to produce high purity tetrabromobisphenols is required, that being, one without the excessive co-formation of alkyl bromides.
U.S. Pat. No. 3,029,291 to Dietzler and U.S. Pat. No. 3,234,289 to Hennis disclose processes to form "higher purity" alkylidenebis (dibromophenols). However, "higher purity" in these early references, for example, is a product of 4,4'-isopropylidene bis(2,6-dibromophenol) with a melting point between 173.degree. C. to 178.degree. C. or a believed purity in excess of 95%. These melting points are likely correct, but the statements of purity are highly suspect given the advances in analytical chemistry since their publication. (Twenty-five years ago this purity data was likely derived using infra-red spectroscopy and estimates of the correlation between melting point and purity. Such a procedure is comparatively less accurate than modern analytical techniques.) Regardless, 4,4'-isopropylidene bis(2,6-dibromophenol) with melting points between 173.degree. C. and 178.degree. C. is commercially unacceptable today. These compounds are now needed with a purity greater than 98% or require melting points greater than 180.degree. C. Yet, the processes disclosed in '291 and '289 only obtain this purity level upon recrystallization, and neither concerns a reduction of alkyl bromide formation which, by estimate, could be produced in these processes at a rate as high as 0.5 Kg per 1 Kg of alkylidene bis(dibromophenol) product.